GaN semiconductor is promising for the material of future high power switching devices owing to its high saturation velocity and breakdown electric field. So far, AlGaN/GaN hetero-junction field effect transistor (HFETs) have been demonstrated with both a low specific on-state resistance and high breakdown voltage taking advantage of inherent high sheet carrier density at the hetro-interface caused by built-in polarization field. Further, as substrate materials, GaN wide-gap semiconductor materials can be grown on Si substrates that can be increased in area with a high thermal conductivity and low cost, as well as conventionally used sapphire and SiC substrates. Thus GaN semiconductor has a number of advantages to power devices.
Referring to FIG. 8, the following will describe an example of a reported field effect transistor using a nitride semiconductor.
FIG. 8 is a sectional view showing the structure of the conventional field effect transistor using a GaN semiconductor.
In FIG. 8, reference numeral 801 denotes a SiC substrate, reference numeral 802 denotes an AlN buffer layer, reference numeral 803 denotes a first undoped GaN layer, reference numeral 804 denotes a first AlGaN layer, reference numeral 805 denotes a Ti/Al drain electrode, reference numeral 806 denotes a protective film, reference numeral 807 denotes a Ni/Au gate electrode, and reference numeral 808 denotes a Ti/Al source electrode. In this structure, the AlN buffer layer 802, the first undoped GaN layer 803, and the first AlGaN layer 804 are formed in this order on the SiC substrate 801, the Ti/Al source electrode 808, the Ti/Al drain electrode 805, and the Ni/Au gate electrode 807 are formed on the first AlGaN layer 804, the protective film 806 made of SiO2 and SiN is formed so as to cover the Ni/Au gate electrode 807, the Ti/Al drain electrode 805, and a part of the Ti/Al source electrode 808, and a part of the Ti/Al source electrode 808 penetrates the protective film 806 and is exposed on the protective film 806. With this structure, it is possible to achieve afield effect transistor having a breakdown voltage of 600 V, a current density of 850 A/cm2 between the source and drain, and a specific on-state resistance (RonA) of 3.3 mΩcm2.
However, as a device for power control, normally off operation is demanded which interrupts current between the source and drain at a gate voltage of 0 V as in Si power MOS transistors. The conventional AlGaN/GaN HFET on the SiC substrate performs a normally-on operation that passes current between the source and drain at the gate voltage of 0 V.
When the normally-on type device is applied to the device of power control circuit, the circuit may be broken due to short-circuit. Thus normally-off type having a positive threshold voltage is strongly required.
In order to achieve a normally-off operation having a positive threshold voltage, it is necessary to reduce the sheet carrier concentration of electrons in a channel. However, in this case, there is a problem that the specific on-state resistance is increased.